fix a possible cause of NaNs

src/main/java/eu/jonahbauer/raytracing/render/renderer/SimpleRenderer.java

@@ -180,7 +180,13 @@ public final class SimpleRenderer implements Renderer {
                     } else {
                         var mixed = new MixtureProbabilityDensityFunction(new TargetingProbabilityDensityFunction(hit.position(), scene.getLights()), pdf, 0.5);
                         var direction = mixed.generate(random);
-                        var factor = pdf.value(direction) / mixed.value(direction);
+
+                        var idealPdf = pdf.value(direction);
+                        var actualPdf = mixed.value(direction);
+                        if (actualPdf == 0) break; // when actualPdf is 0, the ray should have never been generated by mixed.generate
+
+                        var factor =  idealPdf / actualPdf;
+
                         attenuation = Color.multiply(attenuation, Color.multiply(a, factor));
                         ray = new Ray(hit.position(), direction);